Teamkilling in games originated by people exploiting game servers that accepted 'friendly fire'.

More recently, celebrity gaming figures like LOLMAN have utilised impact circles of weapons so as to become profficient at teamkilling, even without 'friendly fire' enabled. In games like Unreal Tournament, this works by firing a projectile in the right place, so that the force of impact makes a teammate fly so far that he loses a frag due to fall damage. This is a 'teamkill'.

A person who, during an internet computer game, kills or damages their team mates intentionally in order to either help the other team (to which they may also be referred to as 'spies') or just to be a cock sucker and/or n00b.
May be abbreviated to TK'er.

Ofter a person in an online internet game who kills their own team mates for the "lolz". In other terms they may do so because they find their team mate inadequate at gaming, in which case they will team kill nubbies.

"Why are you killing your whole team?"
"Just for the lolz."
"WOW you're such a TEAM KILLER"

Or

"I Team Kill Nubs"
"Dude we're only on easy."
"Bounce back to nub land.:
"Don'te be such a TEAM KILLER come on!"

In a FPS videogame (like Call of Duty), someone who kills people on his own team.

I do it in Call of Duty often (how do you think i chose this name?), it really rocks to see people get so pissed off.

Some tips on teamkilling in Call of Duty:

-Always use SMG's or AR's, Rifles are rarley useful in TK'ing, since you are almost always in close proximity.

-In Team Deathmatch mode, when you spawn, back up a few steps and wait. Your TM's should spawn very soon, and you can kill them as they appear. This pisses them off beyond belief.

-FORGE ALLIANCES!!!! If there's another teamkiller on, make sure you ally yourself with him and say on the same team, killing all other TM's and sparing each other.

-Never, ever teamkill with admins on, it's a very nice way to get perma-banned. Make sure you do it when only peons are in the server

-There's also another little strategy I use. (Team wounding) Use an SMG, and shoot your TM's in the legs, enough to ALMOST kill them, but not quite. This way, there's no proof, and they might kill you and then get banned!